fix(bridge): remove dead _enable_ht_attention and its exclusive helpers

[sohv]

Description

Follow-up to #1602, as agreed in #1601 - same defect pattern, same origin commit, separate PR.

ArchitectureAdapter._enable_ht_attention dispatched on three attention layouts, but two of the three branches called methods that do not exist anywhere in the tree:

Branch	Attention layout	Helper called	Defined?
c_attn	GPT-2	_extract_qkv_gpt2_style	yes
q_proj / k_proj / v_proj	Llama, Mistral, Qwen, Gemma, Phi, …	_extract_linear_ht_format	no
query_key_value	GPT-NeoX, Falcon	_extract_qkv_neox_style	no

3efbd6e ("Cleanup (#1129)", 2025-11-15) deleted both helper definitions while keeping their call sites:

$ git show 3efbd6e -- transformer_lens/model_bridge/architecture_adapter.py | grep '^-.*def _extract'
-    def _extract_qkv_neox_style(self, query_key_value, n_heads, d_model, d_head):
-    def _extract_linear_ht_format(self, linear_module, n_heads, d_head, d_model):

Four # type: ignore[attr-defined] comments suppressed the resulting mypy errors, so CI stayed green. Only the GPT-2 branch could run; the other two raised AttributeError on entry.

No user was affected. The same commit also removed both callers (architecture_adapter.py:1249 and bridge.py:1144 as of 3efbd6e^), leaving the method unreachable — verified by searching *.py / *.ipynb / *.md and checking for dynamic getattr dispatch. The risk was latent: the method advertises itself as "architecture-agnostic" in its docstring, so the next person wiring it into component testing would have hit an immediate crash on every architecture except GPT-2.

What is removed

The whole unreachable cluster — 98 lines, the tail of the file:

• _enable_ht_attention
• _extract_qkv_gpt2_style, _extract_output_proj, _disable_hook_conversions — each called only from _enable_ht_attention, so they die with it. _disable_hook_conversions was already a no-op (pass, with a docstring saying it "currently does nothing in no_processing mode").
• the four # type: ignore[attr-defined] comments
• import einops, now unused (pruned by make format)

setup_component_testing and _wire_rotary_for_testing are untouched — they were never part of this path.

If you would rather keep the capability

The opposite fix is equally available: restore _extract_linear_ht_format and _extract_qkv_neox_style from 3efbd6e^ and re-wire a caller. That is a feature with its own design questions (which call path owns it, how it relates to setup_component_testing), so I have not assumed it — happy to send that version instead if you prefer.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have commented my code, particularly in hard-to-understand areas — this is a pure deletion; rationale is in the commit message and above
• I have made corresponding changes to the documentation — no documented behaviour changes; the removed code was unreachable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works — nothing to test: the code is unreachable and removed. Dropping the # type: ignore comments leaves the CI type-check job as the guard against reintroduction.
• New and existing unit tests pass locally with my changes
• I have not rewritten tests relating to key interfaces which would affect backward compatibility

[jlarson4]

Looks great, thank you @sohv!

[sohv]

Great @jlarson4 , happy to contribute!